1. Field of the Invention
The subject application relates to fuel delivery systems, and more particularly, to a fuel pump for use in conjunction with gas turbine engines.
2. Background of the Related Art
Conventional fuel delivery systems utilize a fuel pump to transfer fuel from a storage tank or reservoir to an engine. Gas turbine engines used in aircraft require the fuel pump to supply the fuel at a high pressure. Limitations inherent in the design of some aircraft, such as helicopters where the engines are located several feet above the fuel tank, result in the delivery of fuel from the reservoir to the inlet of the fuel pump at a relatively low pressure. As a result, the fuel pump used in aircraft applications must be capable of operating under low inlet pressure conditions while supplying fuel at the required high pressure.
In an effort to meet the performance demands placed on aircraft fuel delivery systems, the practice of using an inlet pressure boost pump in conjunction with a main fuel pump has been developed. Typically, the main fuel pump is a high pressure pump such as a gear pump. The main fuel pump receives fuel from the inlet boost pump and supplies high pressure fuel to the gas turbine engine. The boost pump is a low pressure pump which receives fuel from the supply reservoir or fuel tank, increases the pressure of the fuel, and then discharges the fuel to the inlet of the main fuel pump. The function of the boost pump is to adequately charge the high-pressure pump even when the boost pump is subjected to poor inlet conditions such as low Net Positive Suction Pressure (NPSP) and/or high Vapor to Liquid (V/L) ratio.
NPSP corresponds to the absolute pressure of the fuel or liquid at the pump inlet expressed in feet of liquid, plus velocity head, minus the vapor pressure of the fluid at pump temperature, and corrected to the elevation of the pump centerline in the case of horizontal pumps or to the entrance of the impeller for vertical pumps. NPSPrequired is determined by the pump manufacturer and is a function of pump speed and pump capacity. NPSPavailable represents the energy level of the fluid over the vapor pressure at the pump inlet and must be at least equal to the sum of the resistances to flow as follows: (1) the vapor pressure of the liquid in the pump chamber; (2) the suction lift when the liquid level is below the pump level; (3) the pressure required to lift the suction valve and overcome the resistance of its spring; (4) the liquid friction in the suction pipeline; (5) the forces required to accelerate the liquid in the suction pipeline; and (6) hydraulic losses in the pump. Unless the NPSPavailable is at least equal to the NPSPrequired during any operating condition, cavitation will occur. The V/L ratio corresponds to a two-phase inlet flow and equals the ratio of vapor to liquid fuel.
For fixed wing aircraft, a typical minimum NPSP value is 5.0 psid and a typical value for the maximum V/L ratio is 0.45. These requirements are often satisfied with a simple boost pump design that includes an inducer and a centrifugal impeller.
In recent years, side channel pumps such as the model EMC-91 boost stage pump manufacture by Chandler Evans Control Systems of West Hartford, Conn. or similar pumps to that shown in U.S. Pat. No. 4,804,313, which is herein incorporated by reference, have been used as boost-stage pumps in aircraft fuel delivery systems, because they have several performance, size, and weight features attractive to these demanding applications. In particular, side channel pumps perform well under adverse inlet conditions, such as low NPSP and high V/L ratio. Additionally, side channel pumps are self-priming. Thus, they are able to pump large air bubbles without having an adverse effect on pumping efficiency or fluid pressure. Air bubbles are a common problem in helicopter applications as a result of the engines being located approximately six feet above the fuel tank.
However, state-of-the-art helicopter applications have increased the demand on the boost pump and require the pump to handle a bubble mixture flow and an alternating liquid/air flow containing air bubbles as long as twelve inches. This corresponding to an NPSP as low as 1.0 psid and V/L ratio as high as 1.0. Although these requirements can be achieved with conventional side-channel pumps, obtaining these performance goals is a difficult proposition, and when achieved, very little performance margin is available.
The operation of conventional side channel pumps is well understood by those skilled in the art. In general, the fuel enters the pump chamber through side entrance port(s) which axially direct fuel flow into the impeller. The rotation of the impeller within the chamber creates a forced vortex flow pattern therein. Typically, two side channels are adjacent to the rotor chamber about an arc centered at zero degrees. Within this arc, circulating flow enters the channels and establishes a helico-toroidal flow pattern. As a result, the fluid passes through the impeller blades a number of times on its path from the inlet region to the discharge region. Each passage through the blades may be regarded as a conventional stage of head generation, and therefore the equivalent pressure rise of a multi-stage pump is achieved in one revolution of the rotor.
In order to maximize the performance of a pumping element such as a side channel pump, it is important for fuel to enter the pumping element at the lowest possible velocity. Generally, the angular velocity of a rotating element, such as a pump rotor or impeller, is directly proportional to the distance from the center of rotation. Therefore, the lowest angular velocity of a rotating impeller blade, is located at the base of the blade and the highest velocity occurs at the blade tip.
As stated, conventional side channel pumps supply fuel axially through an inlet port(s) disposed within the side of the pump housing, parallel to the axis of rotation. Thus, the supplied fuel has to pass the rotor blades at a velocity proportional to the distance between the port and the center of rotation. This results in a degradation of NPSP and V/L performance because of the high blade speed, especially at the outermost radius of the inlet port.
Another problem associated with conventional side-channel pump design is that the configuration of the impellers is less than optimal, from a performance perspective. More specifically, side channel pumps commonly utilize paddle-wheel type impellers or impellers having blades which are for the most part two-dimensional and positioned radially perpendicular to the impeller rotation. This type of blade is typically selected because it is easy to manufacture. However, NPSP and V/L performance is dependent on incidence angle between the blade surface and the direction of the inlet fuel flow. Therefore, the performance of a paddle-wheel impeller is less than optimal, because the flow entering the pumping chamber axially through the side port(s) is not in angular alignment with the blades.
In response to these difficulties, several NPSP and V/L performance improvements have been made with side channel pumps having impellers designed with blades angled with respect to the direction of rotation, partially rectifying the incidence problem. However, these designs are unpopular because they are difficult and expensive to manufacture.
Another problem associated with conventional side-channel pump configurations is that at times the radial space desired for the inlet port, which is a function of the desired inlet flow rate, and the side channel are greater than the radial space available. As a result, the pump designer is forced to reduce the size of the inlet port and/or side channel below the optimum, corresponding to a reduction in pump performance.
As mentioned previously, the requirement to maximize performance of the fuel pump is married to the goal of achieving lightweight and compact designs in the aerospace industry without sacrificing aircraft performance. Whether a side channel pump is used in the fuel delivery system or another close clearance pump design is selected, pump performance can be improved by minimizing both the axial and radial clearance between the impeller and the inlet port and rotor housing. Clearances between the inlet port(s) and the impeller blades are critical and must be minimized to reduce leak paths. These clearances are typically controlled by two axial thrust bearings. Also, critical to the reduction of leak paths is the axial clearance between the impeller and the pump housing. Standard pump designs utilize two large journal bearings located on each end of the rotor. This arrangement evenly distributes the weight of the rotor and the forces generated by the pumping action between the two bearings. The rotors alignment within the pump housing is controlled by the radial clearance between the inside diameter of the journal bearing and the outside diameter of the rotor shaft. The rotor freely can move within these clearances.
In most rotary pump applications, the inlet area needs to be maximized in order to minimize hydraulic losses due to friction and bending. As a result, the journal bearings tend to be large since the inlet must be accommodated inside of the journal bearings. These large bearings require large clearances which conflict with the need for minimizing the radial clearances in the inlet of a center feed device. Since rotor elements typically float within the clearances of the journal bearings, the clearance between the inlet and the rotor is for the most part equivalent to the bearing clearances.
Additionally, as noted, conventional side channel pumps utilize an axial discharge port located in the side of the pump housing, offset from the central axis. The side discharge port is connected to the fuel line leading to the main pump or engines. If a central discharge port could be provided, the space requirements for the pump could be significantly reduced.
There is a need, therefore, for a new fuel pump configuration which cost effectively improves the NPSP and V/L performance by reducing the velocity and incidence at which the fuel contacts the impeller blades and thereby increases the performance margin available for state-of-the art fuel delivery systems. There is also a need for a fuel pump design which reduces leakage losses and maximizes performance of the aircraft pumping elements by minimizing both the axial and radial clearances between the impeller and the inlet port(s) and rotor housing.
The subject application is directed to a new and useful fuel pump for gas turbine engines, and more particularly, to a side channel fuel pump which includes a pump housing having an interior chamber and a discharge port, a rotor member mounted for rotational movement within the interior chamber, and an inlet post member supported within the pump housing for providing fluid to the interior chamber of the pump housing.
The interior chamber of the pump housing defines a central axis for the pump and laterally opposed arcuate channels extending about the central axis. The rotor member, which is disposed within the interior chamber, has a main body portion that includes circumferentially spaced apart radial vane elements, with each vane element having a radially inner base portion and a radially outer tip portion. The rotor member also has a mounting portion for supporting the rotor member within the interior chamber.
The inlet post member has opposed first and second end portions and defines an inlet passage extending between an inlet port associated with the first end portion and a radial discharge port associated with the second end portion. In operation, fluid is admitted into the inlet passage and is delivered at a first pressure radially to the interior chamber of the pump housing at the base portion of the of vane elements. Once the fluid is received into the interior chamber, rotation of the rotor member within the chamber increases the pressure of the fluid, such that the fluid exits the interior chamber at a second pressure through the discharge port of the pump housing.
Preferably, the discharge port of the pump housing extends axially from the interior chamber and is offset from the central axis of the pump. Additionally, the fuel pump further comprises three bearings for supporting the rotor member and maintaining alignment of the rotor member within the interior chamber. The bearings include a journal bearing operatively associated with the mounting portion for maintaining the radial position of the rotor member, and first and second axial thrust bearings for maintaining the axial position of the rotor member.
It is envisioned that the fuel pump of the subject application further comprises a circumferential biasing means disposed within the interior chamber of the pump housing for biasing the first axial thrust bearing towards the rotor member, so as to promote static equilibrium within the housing and axial alignment of the rotor member. In one embodiment, the circumferential biasing means comprises an annular wave washer. The wave washer can have a sinusoidal or tapered cross section which flattens in order to provide the desired stiffness or adjustment capability. Alternatively, the circumferential biasing means comprises a plurality of helical springs. It is envisioned that, the circumferential biasing means further includes at least one shim element for adjusting a biasing force applied by the circumferential biasing means.
In an embodiment, the fuel pump further includes a circular plate member axially mounted for movement within the interior chamber of the pump housing. The plate member is disposed between the main body portion of the rotor member and the first axial thrust bearing and is adapted to restrict the flow of fluid within the interior chamber of the pump housing.
In an embodiment of the subject invention, the inlet post member is dimensioned and configured in such a manner so that an initial close clearance fit exist between the inlet post and the rotor. During the break-in period of the pump, the rotor machines the outer surface of the inlet post so as to create a running clearance between the two components. Thus, the rotor is not supported on the inlet post. Rather, it is axially supported by the axial thrust bearings.
The subject application is also directed to a fuel pump which includes a pump housing having an interior chamber which defines a central axis for the pump and a discharge port. The interior chamber also defines laterally opposed arcuate channels extending about the central axis. The fuel pump further includes a rotor member mounted for rotational movement within the interior chamber and having a main body portion that includes circumferentially spaced apart radial vane elements. The rotor also includes a mounting portion supporting the rotor member within the interior chamber and having an axial discharge passage extending therethrough.
In this embodiment it is envisioned that an inlet post member is supported within the pump housing. The inlet post member has opposed first and second end portions and defines an inlet passage and a outlet passage. As in the previous embodiment, the inlet passage extends between an inlet port associated with the first end portion and a radial discharge port associated with the second end portion. In this embodiment, an outlet passage is associated with the second end portion and it extends between a radial intake port and an axial discharge port. In a manner similar to that of the previously disclosed embodiment, fluid is admitted into the inlet port and is radially delivered at a first pressure to the interior chamber of the pump, wherein the pressure is increased. The rotor member then increases the pressure of the fluid within the interior chamber. Unique to this embodiment, the fluid exits the pump housing at a second pressure through the outlet passage which is associated with the inlet post member.
It is also envisioned that a single journal bearing is operatively associated with the mounting portion of the rotor, and first and second axial thrust bearings are disposed within the interior chamber of the pump housing for maintaining the axial position of the rotor member along with circumferential biasing means.
The subject application is further directed to a pump housing having an interior chamber and a discharge port, with the interior chamber defining a central axis for the pump. A rotor member is mounted for rotational movement within the interior chamber about the central axis, and the rotor member has a main body portion that includes circumferentially spaced apart radial vane elements and a mounting portion for supporting the rotor member within the interior chamber. A journal bearing is operatively associated with the mounting portion for supporting for the rotor member within the housing, and first and second axial thrust bearings are disposed within the interior chamber of the pump housing for maintaining the axial position of the rotor member within the interior chamber of the pump housing. Circumferential biasing means are disposed within the interior chamber of the pump housing for biasing the first axial thrust bearing towards the rotor member so as to promote static equilibrium of forces within the pump housing.
The subject application is additionally directed to a pump housing having an interior chamber and a discharge port. An impeller is mounted for rotational movement within the interior chamber of the pump housing. The impeller has a main body portion and a cantilevered cylindrical extension portion for supporting the impeller within the interior chamber. The cantilevered cylindrical extension portion has an axial discharge passage extending therethrough. An inlet post member is supported within the pump housing, and it has opposed first and second end portions that define an inlet passage and a outlet passage, respectively. A journal bearing is operatively associated with the cantilevered cylindrical extension portion for supporting for the impeller within the housing. Additionally, first and second axial thrust bearings are disposed within the interior chamber of the pump housing for supporting the impeller. Preferably, an annular wave washer is disposed within the interior chamber of the pump housing for biasing the first axial thrust bearing toward the impeller so as to facilitate static equilibrium within the pump housing by restoring the bending moment exerted by the cantilevered extension portion of the impeller. Also, at least one shim element is provided for adjusting the biasing force applied by the wave washer.
Those skilled in the art will readily appreciate that the disclosure of the subject application provides a new fuel pump configuration which effectively improves the NPSP and V/L performance by reducing the velocity and incidence at which the fuel contacts the impeller blades and thereby increases the performance margin available for state-of-the art fuel delivery systems. The subject disclosure also provides a fuel pump configuration which reduces leakage losses and maximizes performance of aircraft pumping elements by minimizing both the axial and radial clearances between the impeller and the inlet port(s) and rotor housing.
These and other unique features of the fuel pump disclosed herein will become more readily apparent from the following description, the accompanying drawings and the appended claims.